Fix reference to solve_wrapper in Solve method

[djsilcock]

Calling solver.StopSearch() (as opposed to from within the solution callback) does not work:

solver=CPSolver()
def stopper():
    solver.StopSearch()
threading.Timer(30,stopper) #in real life would be some external event
solver.Solve()

because self.__solve_wrapper is never set in Solve method

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[lperron]

Thanks